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Forward Equity - Narrative (Details) (Forward equity sales [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Apr. 30, 2014 Subsequent Event [Member]
Derivative [Line Items]
Notional amount		$ 70.2	$ 252.8
Average forward price	199.19	123.09	198.58
Notional amount settled	$ 15.1

X

- Definition

Derivative, Notional Amount, Settled

+ References

No definition available.

+ Details

Name:	amg_DerivativeNotionalAmountSettled
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageForwardPrice
Namespace Prefix:	us-gaap_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration